The present invention relates to turbine engines, and, in particular, relates to a combustor therein.
Many of the most commonly used fuels, such as heavy hydrocarbons, display physical and chemical properties which require great care in the implementation of a prevaporizing and premixing process. For example, complete vaporizing of commercial number 2 heating oil requires that the fuel temperature be raised to approximately 650.degree. F. Further increases in temperature simply assure the gassified state of the fuel. However, at temperatures as low as 900.degree. F., the vaporized fuel begins to break down, chemically forming molecules of both higher and lower molecular weight than that of the original fuel. The new heavy molecules are highly undesirable components as they cause clogging of the vaporizer fuel passages and injectors and produce high particulate emission levels when they burn. Thus, the range of temperature available for the vaporization process is quite narrow and requires very careful control.
The chemical breakdown (pyrolysis) limit on fuel vaporization temperature leads to a second important problem. Although the fuel may be prevaporized, its heat content is insufficient to prevent some degree of recondensation if it is mixed with a stream of colder air. Although this partial recondensation results in extremely fine fog-like liquid droplets, the combustion properties of the system are degraded in comparison with the all gas-phase process. In addition, agglomeration can take place within the fog to produce larger droplets still and fuel can condense along the walls of the apparatus further degrading the combustion characteristics of the system.
The premixed combustion of prevaporized liquid fuels has been the object of previous inventions. In U.S. Pat. No. 4,008,041, careful control of temperature during the vaporization process was accomplished by utilization of an intermediate heat transfer fluid. In U.S. Pat. No. 4,089,638, the need for an intermediate fluid was eliminated by locating a fuel vaporization coil downstream of the point of flame initiation but sufficiently early in the reaction to avoid subjecting the coil to excessive temperatures.
Numerous studies have shown that low levels of nitrogen oxides (NO.sub.x) and smoke emissions, as well as reduced flame radiation levels, can be achieved with lean premixed-prevaporized (LPP) combustion systems. One prior combustor is a single stage lean premixed-prevaporized combustor with conventional flameholder/reactor. Another is a parallel stage combustor with lean premixed-prevaporized catalytic reactor main stage. Several problems have been identified with this arrangement:
(1) A relatively large number of well metered fuel injection points are needed in order to obtain an initially uniform spatial fuel distribution. The fuel injectors, therefore, tend to be mechanically complex, and prone to fouling, which can cause the spatial uniformity to deteriorate with operation; PA1 (2) At modern high-pressure ratio gas turbine operating conditions, ignition delay times are very short--on the order of 1-2 milliseconds, so potentially destructive combustion in the premixing duct is a concern. The duct must be very short, aerodynamically clean, and flow surges must be avoided; and PA1 (3) A high degree of fuel vaporization must be achieved in order to realize the LPP combustion benefits. Complete vaporization is difficult to obtain without encountering autoignition when current heavier fuels are used. Further, because of dwindling petroleum resources and increased demands, future fuels are expected to have increased boiling ranges, which will make it more difficult to achieve a high degree of vaporization.
The use of a normally gaseous fuel (such as natural gas or propane) largely overcomes these problems, since (1) the gaseous fuel metering orifices are much larger and therefore less prone to fouling, and (2) the premixing duct then only needs to be long enough to achieve the desired degree of fuel-air mixing uniformity.
The present invention is directed toward providing a combustor in which the undesirable characteristics of prior liquid fuel systems are overcome in light of the advantages of a purely gaseous system.